A sunstar is not a filter effect. It is a diffraction phenomenon produced by the aperture blades of the lens when a point light source — the sun, a street lamp, a bare bulb — is included in the frame. The RF 14-35mm f/4L IS USM produces it cleanly. The technique is to use it intentionally rather than stumble into it. The mechanics: at wide apertures, light bends minimally around the aperture blades.